1. Field of the Invention
The present invention relates to a technology for detecting a position of a lens in optical devices.
2. Description of the Related Art
Conventionally, lens tubes in which a variable magnification lens, such as a zoom lens, is arranged adjust magnification by moving the lens in a direction of an optical axis. In recent years, the lens tubes have become increasingly complex due to advances in performance of an optical system. For example, as the magnifications in the variable magnification become increasingly high, the number of lenses and parts housed in the lens tube increases. On the other hand, with a demand for compact-sized devices in the market of surveillance cameras and digital cameras, miniaturization of the lens tubes used in these cameras is demanded.
To miniaturize the lens tube in response to such demands, a lens tube having the following configuration, for example, is provided. The lens tube is configured to include a driving device, such as a motor, that performs a driving operation to move the lens. In such lens tubes, for example, a detection-target unit is attached to a lens guide frame to accurately move the lens using the driving device. The detection-target unit, such as a blocking plate, is used to perform position detection of the lens. In the position detection, the detection-target unit attached to the lens guide frame is detected by a sensor. Thus, a position of the lens is detected when the lens is moved (for example, Japanese Patent Laid-open Publication No. H9-105850).
In the conventional technology described above, an optical sensor is used as the sensor. Incident light to the lens tube may enter the sensor, directly, or indirectly due to diffusion inside the lens tube. As a result, erroneous detection occurs, and a position of the lens is erroneously detected at an unintended position in the lens tube. Thus, malfunction of the lens tube may occur.